The global diffusion of HIV, hepatitis B and hepatitis C among injecting drug users (IDUs) shows both that they engage in substantial travel and that research is needed on their travel patterns. Previous research makes it clear that IDUs travel regionally, nationally, and internationally, but has not adequately described travel patterns nor the factors that influence IDUs' HIV risk behavior when they travel. Through our street-based research with IDUs in New York City, we have developed considerable familiarity with a large group of highly mobile, young IDUs, self-defined as "urban nomads", who travel frequently and widely throughout the U.S. These urban nomads are often homeless, but are conceptually and behaviorally distinct from "locally homeless" drug injectors who are not geographically mobile. The specific aims include both methodological and substantive objectives: (Aim 1) Analytically describe a large sample (n = 800) of "urban nomad" young IDUs, including their demographics, drug use histories, travel histories, HIV risk behaviors and serostatus. Identify factors associated with (a) HIV risk behaviors and (b) frequency of intercity travel. (Aim 2) Identify patterns of drug use and HIV risk behavior during travel by urban nomad IDUs, including numbers of "person-trips" (a trip by a person to another metropolitan area), the geographic distribution of person- trips where HIV risk behaviors do and do not occur, and factors that differentiate person-trips with and without HIV risk behavior. Aims 1 and 2 will be achieved through interviews with 600 urban nomad IDUs recruited in New York City and 200 urban nomad IDUs recruited at collaborating research sites across the country. (Aim 3) Implement and assess a long-distance telephone follow-up interview system for collecting data from urban nomad IDUs in their travels. Subjects will be intensively prepared to maintain telephone contact with researchers. Long-distance telephone interviewing will be supplemented by referrals to local research studies for HIV counseling and testing, by referrals to syringe exchange programs, and by e-mail and web site communication. (Aim 4) Determine potential biases in loss to follow-up in using these techniques. Estimated biases can then be used to adjust estimates of continuing drug use and risk behaviors. Aims 3 and 4 will be achieved through a cohort study of 300 subjects recruited in New York. Drug use and HIV risk behavior among IDUs are often viewed as local phenomena, with drug practices, risk behaviors and HIV epidemics occurring within defined geographic areas. The proposed research will provide data and research technologies for a complementary perspective that sees drug use practices and blood-borne pathogens diffusing within a loosely integrated nation-wide "system" of illicit drug use.